1. Field of The Invention
The subject invention relates to power supplies capable of providing multiple voltage outputs and low electromagnetic interference (EMI) noise for use in television receivers.
2. Description of The Related Art
Typically, in television receivers, a PWM switched-mode power supply is used to supply these multiple output voltages. U.S. Pat. No. 4,788,591 discloses such a PWM switched-mode power supply. However, since the voltage and current waveforms associated with such a supply are substantially square waves, a significant amount of EMI noise is generated in the television receivers. In fact, it has become common practice to install snubber (filter) circuits throughout the power supply circuit in the television receiver to reduce ringing and, consequently, to reduce the radiated EMI noise from the supply.
To reduce EMI noise generated from and eliminate snubber circuits used in switched-mode power supplies, resonant power supplies with soft-switching (i.e., zero-voltage-switching or zero-current switching) have been introduced. FIG. 1 shows the circuit schematic of a multi-resonant and multi-output dc-to-dc converter power supply proposed in the article "DESIGN OF HIGH-DENSITY ON-BOARD SINGLE- AND MULTIPLE-OUTPUT MULTI-RESONANT CONVERTERS", by T. A. Tabisz and F. C. Lee, HFPC, May, 1990 Proceedings, pp. 45-57. Due to resonant phenomenon, currents and voltages present in this converter possess the sine-wave like shape except for the gate-to-source voltage of the MOSFET Q1 and the currents through the output rectifying diodes (D1-D6). Furthermore, all parasitics can be used in the converter design. Therefore, the amount of EMI noise generated in the converter is reduced and the snubber circuits are removed. However, the EMI noise generated in the driving circuit is still significant due to the square-wave voltage across the gate and source terminals of the MOSFET, and, consequently, a pulsating gate current with high di/dt is generated. To reduce the noise from the control circuit and the converter, the whole circuit is enclosed in a metal box for shielding, which is an expensive solution. It should be noted that the simple rectifying scheme used in the converter has potential switching losses and EMI noise problems when the converter is used for high output voltage applications, such as for a TV power supply where one output voltage is 130 volts. These problems are caused by the recovery time of high voltage rectifying diodes.